An example of an historical notion, no longer accepted in science.
"Dante explains how Venus goes round with the sphere which forms the third of the skies, but as this does not quite correspond to the phenomena, another sphere which revolves independently is fixed to the sphere of the third sky, and the planet rides on the back of the smaller sphere (sitting like a jewel there, says Dante), reflecting the light of the sun. But writers varied on this point, and we meet the view that the planet was rather like a knot in a piece of wood, or represented a mere thickening of the material that formed the whole celestial sphere – a sort of swelling that caught the sunlight and shone with special brilliance as a result."
Herbert Butterfield (1957) The Origins of Modern Science 1300-1800 (New Edition: Revised and enlarged). G. Bell and Sons Ltd., London
The issue being addressed here (unconvincingly to a modern mind brought up to understand the planets as moving in ellipses around the Sun) is that observations of the motion of the planets did not seem to match the accepted cosmology – that the planets were embedded in crystalline spheres that were concentric with the earth, and moved in perfect circles. These two (seemingly ad hoc) explanations looked to save the phenomena by making what was seen consistent with what was believed.
The comparisons here are presented as similes: Venus is like a jewel sitting on a subsidiary sphere, or like a knot in wood that stands out form the grain pattern.
Read about examples of science similes
